I've always found that the USF AT guns main weakness was not the penetration (special rounds fixed that), but the fact that it is only a 4 man squad. 4 men squads (that decrew after 3 model losses) at just extremely vulnerable to axis lmg DPS, snipers, and rapid fire mortars.
You forgotten something? Maybe something like PaK40? It is still one shoot wiped by Shermans HE rounds.